Guide RNA Design Improves Dmd Prime Editing Efficiency in Wild-Type Mouse Myoblasts
Background: Optimizing design before correcting disease variants
A study published in Cells optimized guide RNA design in wild-type mouse C2C12 myoblasts and increased DNA editing rates at Dmd targets to as high as 20–21%. Although the research concerned variants associated with Duchenne muscular dystrophy (DMD), it did not demonstrate correction in cells carrying disease variants or restoration of muscle function.
DMD is an X-linked neuromuscular disorder caused by variants in the dystrophin gene. Prime editing introduces base substitutions or insertions and deletions without creating DNA double-strand breaks and is being studied as an approach to precisely alter pathogenic variants. But is selecting an editor that can access the target enough? Guide RNA sequence and configuration also influence editing efficiency.
The researchers compared engineered prime editing guide RNAs (epegRNAs) at Dmd regions corresponding to the mdx-4cv and mdx-5cv variants. They used wild-type C2C12 cells carrying neither disease variant and focused on evaluating guide designs and editor combinations.
Key findings: Combining TTCT removal with design changes
The initial nine epegRNAs for the 4cv target achieved editing rates of no more than about 2%. Their reverse transcription templates (RTTs) all contained 5′-TTCT-3′, using the sequence notation in the paper. The researchers redesigned the guides after considering that this sequence might interfere with transcription when guides are expressed from a U6 promoter.
For 4cv optimization, they removed TTCT and introduced a synonymous substitution that changed the protospacer-adjacent motif (PAM) from AGG to CGC. Both codons encode arginine. The epegRNA-16-10 design, with a 16-nucleotide RTT and a 10-nucleotide primer-binding site (PBS), yielded editing rates of 20% with PE3 and 6% with PE6a after a single electroporation.
What was measured here was the proportion of the synonymous PAM variant introduced into wild-type cells. The 20% result should therefore not be interpreted as the actual correction rate for the pathogenic 4cv variant.
For the 5cv target, no suitable NGG PAM was available nearby, so the researchers used the SpCas9-VQR variant, which recognizes an NGAG PAM. Initial editing rates were 2–6%; after redesign, they reached up to 21% with PE-VQR and 9% with PE-SpG. The design that removed TTCT included an E→Q amino acid substitution, changing glutamate to glutamine. Unlike the synonymous substitution for 4cv, this approach also changes the protein sequence.
Editing rates were estimated by Sanger sequencing DNA extracted 72 hours after electroporation and analyzing the results with EditR. These measurements reflect DNA changes at the target site, not the rate of dystrophin protein restoration.
Implications and outlook: Distinguishing the causes of efficiency gains from therapeutic potential
These results provide a basis for considering TTCT presence, RTT and PBS lengths, and PAM changes together when designing U6-driven epegRNAs. However, several elements were changed simultaneously, making it difficult to attribute the entire efficiency gain to TTCT removal alone. The paper also acknowledges the effects of the combined design changes.
Premature transcription termination is an explanation proposed by the researchers. No experiments directly measuring RNA abundance or transcription termination were identified in the supplied full text, so this cannot be described as a directly established mechanism.
The study involved neither patients nor live animals. Correction in cells carrying the actual disease variants, dystrophin expression and function in differentiated myotubes, and in vivo delivery and safety remain to be evaluated. The functional effects of the E→Q substitution accompanying the 5cv design also require examination. This design provides a starting point for follow-up experiments, but it is not a definitive rule applicable to all prime editing targets.
Study conditions and results are available in the full Cells paper.
Duchenne muscular dystrophy (DMD) is a fatal X-linked neuromuscular disorder caused by mutations in the dystrophin gene. Prime editing is a versatile genome editing technology capable of introducing precise nucleotide changes without generating double-strand DNA breaks, making it a promising approach for correcting pathogenic point mutations. In this study, we applied prime editing to modify mdx-4cv and mdx-5cv mutation-equivalent sites in mouse C2C12 myoblasts in vitro. Initial editing efficiencies were unexpectedly low and were associated with the presence of a 5'-TTCT-3' motif within engineered prime editing guide RNAs (epegRNAs). epegRNA designs containing this motif exhibited reduced prime editing efficiency, whereas silent substitution eliminating the motif significantly improved editing outcomes, indicating that specific sequence features within epegRNAs can influence editing performance. Rational redesign of epegRNAs to remove this motif substantially enhanced editing efficiency, achieving up to 20% modification at the 4cv target site using an NGG PAM and 21% editing at the 5cv locus using an NGAG PAM. These findings highlight an important sequence-dependent constraint in epegRNA design and provide practical guidance for optimizing prime editing strategies targeting
Researchers developing prime editing for Dmd targets can use these guide and editor combinations as candidates for comparison in disease models. This involves examining whether U6-driven epegRNAs contain TTCT while comparing RTT and PBS lengths and PAM changes. The differences between the designs must be considered: the 4cv experiments used a synonymous PAM variant as the editing readout, whereas the 5cv design included an E→Q amino acid substitution. Candidates must subsequently be applied to cells carrying the actual variants to assess correction and dystrophin expression and function in differentiated myotubes. DNA editing rates of up to 20–21% in wild-type cells alone cannot establish therapeutic benefit or clinical efficacy.